transmute fatal errors into SYS_ASSERTION_FAILED exit code.

[raulk]

Closes #508.

This PR transmutes fatal errors into the SYS_ASSERTION_FAILED exit code at the top layer (Executor) to produce a consensus outcome on fatal errors.

It also modifies the Backtrace components to generalise them so they can tackle syscall errors as well as fatal errors. The cause is informed at different times for each, so the APIs have been adapted accordingly:

• For syscall errors, we inform the cause as soon as the syscall fails. Thus we reset the backtrace and accumulate the propagation chain (Backtrace#begin()).
• For fatal errors, we inform the cause at the top. Thus we accumulate the propagation chain, then we inform the cause (Backtrace#set_cause()).

I think capturing backtraces (both FVM backtraces and Rust backtraces) is crucial for debuggability. Because I had no way of generating fatal errors to verify the behaviour, I added an integration test which performs no assertions, but prints the backtrace on stdout.

Backtraces from syscall-motivated aborts look like this:

message failed with backtrace:
00: f010000 (method 1) -- actor aborted with code 1 (9)
--> caused by: ipld::open -- failed to parse cid: CIDv0 requires a DagPB codec (1: illegal argument)

Whereas fatal error backtraces display the error chain from anyhow, as well as the backtrace if captured:

fatal backtrace: message failed with backtrace:
00: f010001 (method 1) -- fatal error (10)
--> caused by: [FATAL] Error: [from=f1d3nehuc4u3l5mn7hazppnogf3oe6l6ymaicbkhi, to=f010001, seq=1, m=1, h=0]: missing state: baeaikaia, Stacktrace:
   0: std::backtrace_rs::backtrace::libunwind::trace
             at /rustc/cb121987158d69bb894ba1bcc21dc45d1e0a488f/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1: std::backtrace_rs::backtrace::trace_unsynchronized
             at /rustc/cb121987158d69bb894ba1bcc21dc45d1e0a488f/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2: std::backtrace::Backtrace::create
             at /rustc/cb121987158d69bb894ba1bcc21dc45d1e0a488f/library/std/src/backtrace.rs:328:13
   3: std::backtrace::Backtrace::capture
             at /rustc/cb121987158d69bb894ba1bcc21dc45d1e0a488f/library/std/src/backtrace.rs:296:9
   4: anyhow::error::<impl anyhow::Error>::msg
             at /Users/raul/.cargo/registry/src/jackfan.us.kg-1ecc6299db9ec823/anyhow-1.0.56/src/error.rs:81:36
   5: <fvm::kernel::default::DefaultKernel<C> as fvm::kernel::BlockOps>::block_open::{{closure}}
             at /Users/raul/W/pl/fvm/fvm/src/kernel/default.rs:304:28
   6: core::option::Option<T>::ok_or_else
             at /rustc/cb121987158d69bb894ba1bcc21dc45d1e0a488f/library/core/src/option.rs:1067:25
   7: <fvm::kernel::default::DefaultKernel<C> as fvm::kernel::BlockOps>::block_open
             at /Users/raul/W/pl/fvm/fvm/src/kernel/default.rs:299:20
   8: fvm::syscalls::ipld::open
             at /Users/raul/W/pl/fvm/fvm/src/syscalls/ipld.rs:10:22
   9: core::ops::function::Fn::call
             at /rustc/cb121987158d69bb894ba1bcc21dc45d1e0a488f/library/core/src/ops/function.rs:77:5
  10: <wasmtime::linker::Linker<fvm::syscalls::InvocationData<K>> as fvm::syscalls::bind::BindSyscall<(A,),Ret,Func>>::bind::{{closure}}
             at /Users/raul/W/pl/fvm/fvm/src/syscalls/bind.rs:171:44
  ...

[Stebalien]

One nit, but feel free to ignore it.

[Stebalien]

So much nicer.

[Stebalien]

We can collapse these into a single enum.

/// The ultimate "cause" of a failed message.
#[derive(Clone, Debug)]
pub enum Cause {
    /// The original cause was a syscall error.
    Syscall {
        module: &'static str,
        /// The syscall function name.
        function: &'static str,
        /// The exact syscall error.
        error: ErrorNumber,
        /// The informational syscall message.
        message: String,
    },
    /// The original cause was a fatal error.
    Fatal {
        /// The alternate-formatted message from the anyhow error.
        error_msg: String,
        /// The backtrace, captured if the relevant
        /// [environment variables](https://doc.rust-lang.org/std/backtrace/index.html#environment-variables) are enabled.
        backtrace: String,
    },
}

[raulk]

I had inline structs in a previous version, but promoted them to types so I could annotate them with derive macros. I think I don't need those now, so I can demote them again.

[arajasek]

LGTM